In a spread spectrum system, a transmitted signal is spread over a frequency band that is much wider than the minimum bandwidth required to transmit particular information. Whereas in other forms of modulation, such as amplitude modulation or frequency modulation, the transmission bandwidth is comparable to the bandwidth of the information itself, a spread spectrum system spreads an information bandwidth of, for example, only a few kilohertz over a band that is many megahertz wide, by modulating the information with a wideband encoding signal. Thus, an important characteristic distinguishing spread spectrum systems om other types of broadband transmission systems is that inspread spectrum signal processing, a signal other than the information being sent spreads the transmitted signal.
Spreading of the transmitted signal in typical spread spectrum systems is provided by (1) direct sequence modulation, (2) frequency hopping or (3) pulsed-FM or "chirp" modulation. In direct sequence modulation, a carrier is modulated by a digital code sequence whose bit rate is much higher than the information signal bandwidth. Frequency hopping involves shifting the carrier frequency in discrete increments in a pattern dictated by a code sequence, and in chirp modulation, the carrier is swept over a wide band during a given pulse interval. Other, less frequently used, carrier spreading techniques include time hopping, wherein transmission time, usually of a low duty cycle and short duration, is governed by a code sequence and time-frequency hopping wherein a code sequence determines both the transmitted frequency and the time of transmission.
Applications of spread spectrum systems are various, depending upon characteristics of the codes being employed for band spreading and other factors. In direct sequence spread spectrum systems, for example, wherein the code is a pseudo-random sequence, the composite signal acquires the characteristics of noise, making the transmission undiscernable to an eavesdropper who is not capable of decoding the transmission. Additional applications include navigation and ranging with a resolution depending upon the particular code rates and sequence lengths used. Reference is made to the textbook of R. C. Dixon, Spread Spectrum Systems, John Wiley and Sons, New York, 1976, especially Chapter 9, for application details.
Direct sequence modulation involves modulation of a carrier by a code sequence of any one of several different formats, such as AM or FM, although biphase phase-shift keying is the most common. In biphase phase shift keying (PSK), a balanced mixer whose inputs are a code sequence and an RF carrier, controls the carrier to be transmitted with a first phase shift of X.degree. when the code sequence is a "1" and with a second phase shift of (180+X).degree. when the code sequence is a "0". Biphase phase-shift keyed modulation is advantageous over other forms because the carrier is suppressed in the transmission making the transmission more difficult to receive by conventional equipment and preserving more power to be applied to information, as opposed to the carrier, in the transmission. Characteristics of biphase phase-shift keying are given in Chapter 4 of the aforementioned Dixon text.
The type of code used for spreading the bandwidth of the transmission is preferably a linear code, particularly if message security is not required, and is a maximal code for best cross correlation characteristics. Maximal codes are, by definition, the longest codes that could be generated by a given shift register or other delay element of a given length. In binary shift register sequence generators, the maximum length (ML) sequence that is capable of being generated by a shift register having n stages is 2.sup.n -1 bits. A shift register sequence generator is formed from a shift register with certain of the shift register stages fed back to other stages. The output bit stream has a length depending upon the number of stages of the register and feedback employed, before the sequence repeats. A shift register having five stages, for example, is capable of generating a 31 bit binary sequence (i.e. 2.sup.5 -1), as its maximal length (ML) sequence. Shift register ML sequence generators having a large number of stages generate ML sequences that repeat so infrequently that the sequences appear to be random, acquiring the attributes of noise, and are difficult detect. Direct sequence systems are thus sometimes called "pseudo-noise" systems.
Properties of maximal sequences are summarized in Section 3.1 of Dixon and feedback connections for maximal code generators from 3 to 100 stages are listed in Table 3.6 of the Dixon text. For a 1023 bit code, corresponding to a shift register having 10 stages with maximal length feedback, there are 512 "1"s and 511 "0"s; the difference is 1. Whereas the relative positions of "1"s and "0"s vary among ML code sequences, the number of "1"s and the number of "0"s in each maximal length sequence are constant for identical ML length sequences.
Because the difference between the number of "1"s and the number of "0"s in any maximal length sequence is unity, autocorrelation of a maximal linear code, which is a bit by bit comparison of the sequence with a phase shifted replica of itself, has a value of -1, except at the 0.+-.1 bit phase shift area, in which correlation varies linearly from -1 to (2.sup.n -1). A 1023 bit maximal code (2.sup.n -1) therefore has a peak-to-average autocorrelation value of 1024, a range of 30.1 db.
It is this characteristic with makes direct sequence spread spectrum transmission useful in code division multiplexing. Receivers set to different shifts of a common ML code are synchronized only to transmitters having that shift of the common code. Thus, more than one signal can be unambiguously transmitted at the same frequency and at the same time. In an autocorrelation type multiplexed system, there is a common clock or timing source to which several transmitters and at least one receiver are synchronized. The transmitters generate a common miximal length sequence with the code of each transmitter phase shifted by at least one bit relative to the other codes. The receiver generates a local replica of the common transmitted maximal length sequence having a code sequence shift that corresponds to the shift of the particular transmitter to which the receiver is tuned. The locally generated sequence is autocorrelated with the incoming signal by a correlation detector adjusted so as to recognize the level associated with only .+-.1-bit synchronization to despread and extract information from only the signal generated by the predetermined transmitter.
Because the autocorrelation characteristic of a maximal length code sequence has an offset corresponding to the inverse of the code length, or EQU V/(2.sup.n -1)
where V is the magnitude of voltage corresponding to "1" and n is the number of shift register stages, overlap occurs in neighboring channels. Thus, there is imperfect rejection of unwanted incoming signals. Unambiguous signal discrimination thus requires a guard band between channels reducing the number of potential transmitters for a given code length. A long maximal length sequence compensates for the guard band to increase the number of potential transmitters, but this slows synchronization and creates power imbalance of the multiplexing transmitters.
In such systems, synchronization of the receiver to the predetermined transmitter is performed in stages. First, there is static delay of receiver timing to compensate for fixed variations in synchronization timing between the receiver and the predetermined transmitter. Static delay can be determined based upon the distance between the transmitter and receiver and the characteristics of the medium, e.g., transmission line, between them, or can be simply measured to synchronize the receiver and predetermined transmitter to be within plus or minus one code chip of each other (a code chip is defined as a bit period of the pseudo-random code generator within the direct sequence spread spectrum system).
Second, variable delays, which are unknown, are compensated within the receiver by a variable, or dynamic, delay that is controlled in two steps, fine tuning and coarse tuning. Fine tuning of the receiver is limited to a range of plus or minus a portion of a code chip from a predetermined point which may be at the last correct location of a message received. Fine tuning causes the receiver to lock onto a local peak when the signal is present. There are several different methods by which fine tuning is accomplished, such as through serial hunting, using a code preamble to reduce lock-on time. Coarse tuning, operative after the receiver has been fine tuned at a local peak determines whether the local peak is the "correct" local peak for best correlation, in other words, whether the receiver and predetermined transmitter are both locked onto the same system of clock signals. If the receiver and transmitter are synchronized to different clock signals, the receiver will lock onto an improper correlation peak. During coarse tuning, the receiver tests correlations of the receiver timing with neighboring correlation peaks and selects the largest, based upon maximum signal-to-noise ratio.
In a communication system of this type, as well as of other types, it is desirable to optimize system performance by maximizing signal-to-noise ratio and receiver-to-transmitter synchronization. Because multiple channels, each containing a sub-receiver with a correlation detector, exist in a system of the above described type, information is available that is not presently used. For example, the correlation properties of the code employed as a function of code chip delay differences between received and reference codes has a peak when synchronization is achieved with an absolute value dropping to "zero" as synchronization difference approaches a code chip or greater. The sign of the pattern is dependent upon the data bit being used to modulate the transmitter. By monitoring the sign of the correlation output when the receiver is properly synchronized to the transmitter, it is possible to recover the transmitted data. With each sub-receiver tuned to a different correlation peak, data are present at each and are available to be processed to enhance the signal-to-noise ratio of the receiver as well as to improve synchronization.